Uptake of hematoporphyrin derivative by normal and malignant cells: effect of serum, pH, temperature, and cell size

Normal and malignant cells were incubated with hematoporphyrin derivative (HPD) and their uptake and retention of HPD were analyzed by flow cytometry. In standard growth medium the amount of HPD taken up by cells was proportional to the added HPD concentration and reached a plateau level after 5-6 h of incubation. The uptake occurred in two steps; within seconds a large amount of HPD became loosely bound to the cells, presumably the outer membrane. This was followed by a slower uptake of HPD into the cytoplasm. The loosely bound portion could be washed from the cells by medium containing either fetal calf serum or serum albumin. At low temperatures the uptake into the cytoplasm was strongly reduced. A major determinant of HPD uptake was the concentration of serum in the medium. At any particular concentration of HPD below 200 mg/liter, increasing concentrations of fetal calf serum or bovine serum albumin resulted in a reduction in the amount of HPD taken up by cells. A further factor affecting uptake was the pH of the medium. At low pH (pH 6) the rate of HPD incorporation was much higher than at pH 7.4. Under identical conditions of incubation, HPD uptake was proportional to cell size as estimated using the low angle light scatter signal in the flow cytometer. Our data suggest that acidic pH, differences in extracellular serum concentrations of malignant tumor tissue, as well as the increased size of tumor cells may play an important role in the selective uptake of HPD by malignant tumors.